Biomechanics of Intramedullary Nailing

Biomechanics of Intramedullary Nailing :

Biomechanics of I ntramedullary Nailing Dr Anton Priyantha Warnakulasuriya MBBS(SL) MS ( Orth ) Resident NAMS

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The intramedullary nail is commonly used for long-bone fracture fixation and has become the standard treatment of most long-bone diaphyseal and selected metaphyseal fractures. To understand the intramedullary nail, knowledge of evolution and biomechanics are helpful .

History of Intramedullary Nails:

The Beginnings – 16 th Century In Mexico Aztec physicians have placed wooden sticks into the medullary canals of patients with long bone non-union. Mid 1800’s Ivory pegs were inserted into the medullary canal for non-union. 1917 ‘s Hoglund of United States reported the use of autogenous bone as a intramedulary implant. History of Intramedullary Nails

History of Intramedullary Nails:

1931 Smith-Petersen reported the success of stainless steel nails for the treatment of NOF #s 1930’s In the United States, Rush and Rush described the use of Steinman pins placed in the medullary canal to treat fractures of the proximal ulna and proximal femur. History of Intramedullary Nails

History of Intramedullary Nails:

History of Intramedullary Nails 1940 ‘s The Evolution of K ű ntscher Nailing Gerhard K ű ntscher was born in Germany in 1900

History of Intramedullary Nails:

K ű ntscher Nailing V-shaped stainless steel nail was invented by K ű ntscher . it was first used in 1940. By late 1940s, K ü ntscher had designed the cloverleaf nail . History of Intramedullary Nails

History of Intramedullary Nails:

1950’s Two important techniques were developed. Intramedullary reamers Interlocking Screws K ü ntscher . Developed Flexible reamers . History of Intramedullary Nails

History of Intramedullary Nails:

1950’s Interlocking Screws Modny and Bambara introduced the transfixion intramedullary nail in 1953 1960’s with the development of radiological image intensification, Intramedullary nailing was very popular 1970’s and 1980’s Development of slotted cloverleaf-shaped interlocked nail History of Intramedullary Nails

History of Intramedullary Nails:

1990’s and the 21 st Century Slotted cloverleaf designs were being replaced by non-slotted designs. Introduction of new titanium nails, ,Flexible Nails ,third generation nails History of Intramedullary Nails

Various generations of nails :

Consecutive advancements of nails over years Can be grouped under three generations 1 st generation primarily act as splints ,rotational stability is minimal , primarly relies on close fit Eg –K nail , V nail 2 nd generation Improved rotational stability due to locking screw Eg-Russel taylor nail 3 rd generation Nails with various designs to fit anatomocally as much as possible ,to aid the insertion and stability Eg -Nails with multiple curves ,multiple fixation systems Tibial nail with malleolar fixation Various generations of nails

BIOMECHANICS:

When placed in a fractured long bone, IM nails act as internal splints with load-sharing characteristics. Various types of load act on an IM nail: torsion, compression, tension and bending Physiologic loading is a combination of all these forces BIOMECHANICS

BIOMECHANICS:

The amount of load borne by the nail depends on the stability of the fracture/implant construct. This stability is determined by Nail Characteristics Number and orientation of locking screws Distance of the locking screw from the fracture site Reaming or non reaming Quality of the bone IM nails are assumed to bear most of the load initially, then gradually transfer it to the bone as the fracture heals. BIOMECHANICS

BIOMECHANICS:

If cortical contact across the fracture site is achieved postoperatively, most of the compressive loads are borne by the bony cortex; In the absence of cortical contact, compressive loads are transferred to the interlocking screws, which results in four-point bending of the screws BIOMECHANICS

   Nail Characteristics   :

Several factors contribute to the overall biomechanical profile and resulting structural stiffness of an IM nail. Chief among them are Material properties Cross-sectional shape Diameter Curves Length and working length Extreme ends of the nail Supplementary fixation devices Nail Characteristics

Nail Characteristics :

Material properties Construction of IM nails are titanium alloy and 316L stainless steel. Stainless steel has twice of modulus elasticy of cortical bone Titanium alloy has a modulus of elasticity closely approximates that of cortical bone ( Modulus is ability to resist deformation in tension ) Nail Characteristics

Nail Characteristics Cross-sectional shapes Of medullary nails:

A-Schneider B-Diamond C-Sampson fluted D- Kuntscher E-Rush F-Ender G- Mondy H-Halloran I- Huckstep J-AO/ASIF K-Grosse – Kempf L-Russell-Taylor J,K,L-Now commonely used Nail Characteristics Cross-sectional shapes Of medullary nails

Nail Characteristics :

Cross-sectional shape cont. The cross-sectional shape of the nail ,Diameter and the area of the nail determines its bending and torsional strengths( Resistance of a structure to torsion or twisting force is called polar movement of inertia ) Circular nail has polar movement of inertia proportional to its diameter, in square nail its proportional to the edge length Nails with Sharp corners or fluted edges has more polar movement inertia Cloverleaf design resist bending most effectively Presence of slot reduces the torsional strength . It is more rigid when slot is placed in tensile side Nail Characteristics

Nail Characteristics :

Diameter Nail diameter affects bending rigidity of nail. For a solid circular nail, the bending rigidity is proportional to the third power of nail diameter Torsional rigidity is proportional to the fourth power of diameter Nail Characteristics

Nail Characteristics:

CURVES Long bones have curved medularly cavities Nails are contoured to accommodate curves of the bone Average radius of curvature of femur is 120 (±36) cm. Femoral nail designs have considerably less curve, with radius ranging from 186 to 300 cm. Nail Characteristics

Nail Characteristics curves cont:

Nail Characteristics curves cont

Nail Characteristics:

CURVES cont . radiograph demonstrating that mismatch in the radius of curvature between the nail and the femur can lead to distal anterior cortical perforation. Nail Characteristics

Nail Characteristics:

CURVES cont.. Tibial nail also has a smooth 11 bend in the anterioposterior direction at junction of upper one third and lower two third . It is called angle of herzog Nail Characteristics

Nail Characteristics:

CURVES cont. When inserting nail , axial force is necessary as the nail must bend to fit the curvature of the modularly canal The insertion force generates hoop stress in the bone ( Circumferential expansion stress ) Greater the insertion force higher the hoop stress .Larger hoop stress can split the bone Over reaming the entry hole by 0.5-1mm ,selecting entry point posterior to the central axis reduce the hoop stress Nail Characteristics

Nail Characteristics:

Nail Characteristics CURVES /hoop stress cont . Example The ideal starting point for insertion of an antegrade femoral nail is in the posterior portion of the piriformis fossa . It reduces the hoop stress

Nail Characteristics:

Length and working length A-Total nail length- total anatomical length B-Working length- Length of a nail spanning the fracture site from its distal point of fixation in the proximal fragment to proximal point of fixation in the distal fragment -length between proximal and distal point of firm fixation to the bone -Un supported portion of the nail between two major fragments Nail Characteristics B A

Nail Characteristics:

Working length cont.. working length is affected by various factors Type of force (Bending ,Torsion ) Type of fracture Interlocking Reaming Nail Characteristics

Working length cont.. :

For bending loads A nail fixing a transverse fracture has a shorter working length than one fixing a comminuted fracture Working length cont ..

Working length cont..:

For torsional loads when a nail is fixed to the bone by interlocking screws working length equals to the definite points of fixation Working length cont ..

Working length cont..:

Weight bearing interlocking nail In weight bearing interloccking nail bows and Increase the nail bone contact near the fracture site. Increase contact force reduces the working length for bending and torsional force Working length cont..

   Interlocking Screw :

Interlocking screws are recommended for most cases of IM nailing. The number of interlocks used is based on fracture location, amount of fracture comminution , and the fit of the nail within the canal. Placing screws in multiple planes may lead to a reduction of minor movement Interlocking Screw

Interlocking Screw :

Interlocking Screw Static locking- when screws placed proximal and distal to the fracture site. This restrict translation and rotation at the fracture site. Indications – communited , spiral,pathologicalfractures Fractures with bone loss lengthning or shortening osteotomies , Atropic non union

Interlocking Screw:

Dynamic locking – when screws are inserted only at one end(short proximal fragment ) Indications Fractures with good bone contact Non unions Interlocking Screw

Interlocking Screw:

Dynamisation Removal of locking screw,it is indicated when there is nonnunion or pseudoarthrosis screws are removed from long fragment Can be perform within 3 rd month of treatment, It enhans fracture healing Interlocking Screw

Interlocking Screw:

stability depends on the locking screw diameter for a given nail diameter. In general, 4 to 5 mm for humeral nails and 5 to 6 mm for tibial and femoral nails. Nail hole size should not exceed 50% of the nail diameter. Interlocking screws undergo four-point bending loads, with higher screw stresses seen at the most distal locking sites. Interlocking Screw

Interlocking Screw:

Interlocking Screw The number of locking screws is determined based on fracture location and stability. In general, one proximal one distal screw is sufficient for stable fractures.

Interlocking Screw:

The location of the distal locking screws affects the biomechanics of the fracture . The closer the fracture to the distal locking screws, the nail has less cortical contact , which leads to increased stress on the locking screws. More distal the locking screw is from fracture site, the fracture becomes more rotationally stable . Interlocking Screw

Interlocking Screw:

Oblique ( angled to nail axis, not 90°) proximal locking screws appear to increase the stability of proximal tibia fractures compared with transverse ( 90° to nail axis) locking screws. However, oblique or transverse orientation of the distal screws in distal-third tibia fractures has minimal effect on stability. Interlocking Screw

Interlocking Screw:

Orientation of the proximal femur locking screws has little effect on fixation stability, with both oblique and transverse proximal locking screws showing equal axial load to failure. Two screws can be inserted at angles to the cross-section of the nail to decrease motion between the screws and the nail, but anatomic structures must be taken into consideration when performing this technique. Interlocking Screw

Extreme ends of the nail:

K-nail has slot/eye in the either ends for attachment of extraction hook .one end is tapered to facilitate the insertion Present version of cannulated locking screw contains cylinderical proximal end with internally threaded core to allow firm attachment of driver and extracter.Holes for interlocking screws present either ends Some nails have slots near the distal end for placement of anti rotation screw Extreme ends of the nail

Closed and open nailing :

Closed nailing Fluoroscopy is used to achive fracture reduction Medullary cavity is entered through one end of the bone “ antegrade “ eg-Piriformis fossa in femur Closed antegrade nailing is the method of choice Open nailing Performed in lessthan ideal operation room conditions Antegrade nailing is prefered In retrograde method nail is inserted in to the proximal fragment through fracture site and brought out at one end of the bone ,after reduction nail is driven in to the distal fragment Infection and non union is six and ten times greater in open nailing Closed and open nailing

Biomechanics of Intramedullary Reaming :

IM reaming can act to increase the contact area between the nail and cortical bone by smoothing internal surfaces. When the nail is the same size as the reamer, 1 mm of reaming can increase the contact area by 38% Reaming reduces the working length and increase the stability. More reaming allows insertion of a larger-diameter nail, which provides more rigidity in bending and torsion. Biomechanically, reamed nails provide better fixation stability than do unreamed nails. Biomechanics of Intramedullary Reaming

Weight Bearing After Intramedullary Nailing :

Segmentally comminuted diaphyseal fracture without bony contact and nails with a 12-mm diameter and two distal locking bolts could with stand the typical biomechanical forces of weight bearing. In patients who retain diaphyseal bony contact after fracture fixation, nails with a diameter <12 mm or nails with a single distal interlock may provide adequate stability for weight bearing because the bony contact reduces the load encountered by the distal interlocking screws. weight bearing through a locked IM nail could be allowed in fractures in which 50% cortical contact is present. Weight Bearing After Intramedullary Nailing

Intramedullary nail failure :

As with all metallic implants, there is a relative race between bone healing and implant failure. Occasionally, an implant will break when fracture healing is delayed or when nonunion occurs. IM nails usually fail in predictable patterns. Unlocked nails typically fail either at the fracture site or through a screw hole or slot. Locked nails fail by screw breakage or fracturing of the nail at locking hole sites, most commonly at the proximal hole of the distal interlocks Intramedullary nail failure

Intramedullary Nail removal :

It is not necessary to remove a nail in a weight bearing limb unlike a plate. If needed can be removed after 18 months. Indications for removal- Patient request, pain swelling secondary to backing out of the implant. Nail removal should not be undertaken lightly specialized extraction equipment fitting the nail must be available. Full weight bearing can commence immediately after the removal of nail Intramedullary Nail removal

Future Directions :

Nails constructed of biodegradable polymers will provide temporary stabilization of fractures without the potential long-term effects of a retained foreign implant. Nickel-titanium shape-memory alloys may enable the development of implants that can change shape as they warm to patient body temperature. These implants can improve stability as they change shape after insertion and recover curvature as they warm. IM nails coated with biologically active agents, such as bone morphogenetic proteins, could help diminish nonunion rates, while nails coated with antibiotics could potentially limit postoperative infection. Future Directions

References :

The Elements Of Fracture Fixation Anand Thakur Campbell s operative orthopedics Canale Beaty Interlocking nails - Rana Matthew R et al Intramedullary Nailing of the Lower Extremity: Biomechanics and Biology -J Am Acad Orthop Surg 2007;15:97-106 Historical overview and biomechanical principles of intramedullary nailing ztok A. Pilih , Andrej Čretnik References

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THANK YOU Mark D . Millar